We travelled through Spain and France crossing the Pyrenees last summer in the Chummy, 5 up with a trailer in 40c heat. We suffered no vapouristion issues with standard Zenith on gravity feed.

I would suggest your issues lie more with the fuel feed as mentioned by Dave above - s/d carbs really need a pump of some sort to work properly, particularly when being tested on longer inclines.

I don't recall the F/B post mentioned by Sandy - apologies.

Since fitting a four-blade fan to my RP any problems regarding fuel vapourisation have disappeared. The car has a S/D SU and the standard engine driven T type fuel pump. Everything under the bonnet seems hunky-dorey when slogging up long hills although it does get a bit hot in the cabin especially when the ambient temp is over 32°C. I put this down to the greater air flow from the big fan.

The engine does take a little longer to reach operating temp on cooler days, but that's a trade-off I'm happy to accept.

When on the Euro tour going up particularly high and long pass my carb bowl was so hot that it could not be touched, the point where the carb meets the manifold had water droplets at the point of freezing. Definitely evaporation going on . Waited for her to cool down and finished the clump as in photo ( or at least what I hope will be a photo. Oh 1938 Opal , standard v26 carb

Location: Oakley, hants